Turbochargers and charge air compressors are well known and widely used with internal combustion engines. Generally, such devices supply more charge air for the combustion process than can otherwise be induced through natural aspiration. This increased air supply allows more fuel to be burned, thereby increasing power and torque obtainable from an engine having a given displacement. Additional benefits include the possibility of using lower-displacement, lighter engines with corresponding lower total vehicle weight to reduce fuel consumption, and use of available production engines to achieve improved performance characteristics. In some applications an intercooler is incorporated for removing heat (both an ambient heat component and heat generated during charge air compression) from the charge air before it enters the engine, thereby providing an even more dense air charge to be delivered to the engine cylinders. Intercooled turbocharging applied to diesel engines has been known to at least double the power output of a given engine size, in comparison with naturally aspirated diesel engines of the same engine displacement.
Additional advantages of turbocharging include improvements in thermal efficiency through the use of some energy of the exhaust gas stream that would otherwise be lost to the environment, and the maintenance of sea level power ratings up to high altitudes.
At medium to high engine speeds, there is an abundance of energy in the engine exhaust gas stream and, over this operating speed range, the turbocharger is capable of supplying the engine cylinders with all the air needed for efficient combustion and maximum power and torque output for a given engine construction. In certain applications, however, an exhaust stream waste gate is needed to bleed off excess energy in the engine exhaust stream before it enters the turbocharger turbine to prevent the engine from being overcharged. Typically, the waste gate is set to open at a pressure below which undesirable predetonation or an unacceptably high internal engine cylinder pressure may be generated.
At low engine speeds, such as idle speed, however, there is disproportionately little energy in the exhaust stream as may be found at higher engine speeds, and this energy deficiency prevents the turbocharger from providing a significant level of boost in the engine intake air system. As a result, when the throttle is opened for the purpose of accelerating the engine from low speeds, such as idle speed, there is a measurable time lag and corresponding performance delay, before the exhaust gas energy level rises sufficiently to accelerate the turbocharger rotor and provide the compression of intake air needed for improved engine performance. The performance effect of this time lag may be pronounced in smaller output engines which have a relatively small amount of power and torque available before the turbocharger comes up to speed and provides the desired compression. Various efforts have been made to address this issue of time lag, including reductions of inertia of turbocharger rotors.
In spite of evolutionary design changes for minimizing the inertia of the turbocharger rotor, however, the time lag period is still present to a significant degree, especially in turbochargers for use with highly rated engines intended for powering a variety of on-highway and off-highway equipment.
Furthermore, to reduce exhaust smoke and emissions during acceleration periods when an optimal fuel burn is more difficult to achieve and maintain as compared with steady-speed operation, commercial engines employ devices in the fuel system to limit the fuel delivered to the engine cylinders until a sufficiently high boost level can be provided by the turbocharger. These devices reduce excessive smoking, but the limited fuel delivery rate causes a sluggishness in the response of the engine to speed and load changes.
The turbo-lag period can be mitigated and, in many instances, virtually eliminated by using an external power source to assist the turbocharger in responding to engine speed and load increases. One such method is to use an external electrical energy supply, such as energy stored in d.c. batteries, to power an electric motor attached to the turbocharger rotating assembly. The electric motor can be external and attached to the turbocharger rotor through a clutching mechanism, or it can be added onto the turbocharger rotating assembly and energized and de-energized through the use of appropriate electronic controls. For example, a plurality of permanent magnets may be attached to the rotatable shaft of a turbocharger to provide an electric motor rotor and a plurality of stator windings may be incorporated into the turbocharger around the rotor magnets. Energization of the stator windings with polyphase electrical energy can drive the permanent magnet rotor in rotation when additional charge air is needed. Patents disclosing turbocharger-electrical machine combinations include U.S. Pat. Nos. 5,406,797; 5,038,566; 4,958,708; 4,958,497; 4,901,530; 4,894,991; 4,882,905; 4,878,347 and 4,850,193.
The very hot and high centrifugal force environments of turbochargers and charge air compressors are hostile to electrical motor elements, such as permanent magnets and electrically insulated stator windings. The attachment of the permanent magnets to the turbocharger shaft subjects the magnets to heat which is conducted along the shaft from the hot turbine wheel of the turbocharger. This presents a significant problem in that the permeability of the magnets may be reduced by such heating to a level which may be unacceptable for efficient operation of the rotary electric machine. In addition, the very high temperatures generated in operation of a turbocharger endangers the integrity of stator winding electrical insulation. This becomes a serious problem when the turbocharged engine is subjected to a hot shutdown and the oil flow through the bearings and over the shaft is interrupted. High temperatures and steep temperature gradients will exist for a significant length of time while the hot parts of the turbocharger are drained of their heat content. Furthermore, turbochargers and charge air compressors rotate at very high speeds, and frequently at speeds in excess of 60,000 to 80,000 rpm. The machine elements exposed to such very high speeds of rotation are subjected to very high centrifugal forces.